What's Inside Saturn Moon Enceladus? Geyser Timing Gives Hints

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Eruptions of vapor and ice that stretch for miles above the
surface of Saturn's icy moon Enceladus experience a mysterious
delay that could be due to a weak interior, new research
suggests.

The geysers that burst from the surface of Enceladus indicate
that this Saturnian satellite is more than just a ball of ice.
Scientists believe a liquid-water ocean exists beneath the solid
surface, and that life could potentially survive there. A better
understanding of the geysers could provide clues as to what is
going on in that subterranean environment, the study scientists
said.

Previous work has suggested that the geysers are caused by the
tidal pull of Saturn on Enceladus, but the timing of those
explosions with the motions of the two bodies doesn't quite line
up based on models. The new research provides insight into what
kind of internal arrangement might be responsible for the timing
of the geysers. [ Inside
Enceladus, Icy Moon of Saturn (Infographic) ]

Enceladus is Saturn's sixth-largest moon, a 310-mile-wide (500
kilometers) satellite covered in an icy shell. In 2005, NASA's
Cassini spacecraft spotted water vapor and icy particles erupting
from its south pole — explosions that may have their origin in
water from a giant,
hidden ocean.

The geysers on Enceladus vary in how active they are, depending
on where the alien moon is on its slightly "eccentric," or
oval-shaped, orbit, which takes about 1.37 Earth days to
complete. This led scientists to suspect that tidal forces
trigger the eruptions on Enceladus — as the strength of Saturn's
gravitational pull on Enceladus varies over time, so too does the
amount of stress on giant cracks in the alien moon's icy shell,
opening and closing these rifts.

However, the eruptions seen from Enceladus always seemed delayed
by several hours with respect to predictions based on simple
tidal models. To help solve this mystery, scientists developed
more complex 3D models that varied the viscosity of the alien
moon's interior to see how that might influence how Enceladus
behaved.

"Previous predictions were too simple, in that they ignored
important details of the structure of Enceladus,"
study co-author Francis Nimmo, a planetary scientist at the
University of California, Santa Cruz, told Space.com.

The viscosity of a fluid is a measure of how thick it is — how
much it resists flow. For example, water is a relatively
low-viscosity fluid, while honey is a relatively high-viscosity
one.

The scientists found that although a strong, high-viscosity ice
shell would react instantly to tidal forces, a weaker,
low-viscosity ice shell would react more gradually.

"Enceladus experiences tides from Saturn,
which provide a force on the ice shell," Nimmo said. "The ice
shell flows in response to these forces, and because the flow is
quite slow, the response is delayed by several hours."

The researchers propose two potential models that could explain
the delay seen in Enceladus' geysers. One involves a global
subsurface ocean, with an ice shell more than 30 miles (50 km)
thick that churns throughout the entire ocean. The other involves
a subsurface sea limited to the south polar region. In this
model, the churning of the ice shell is limited to an area right
above this sea in a layer that is only about 18 miles (30 km)
thick.

"The timing of geyser activity gives us an insight into the
interior of a rather complicated planetary body," study lead
author Marie Běhounková, a planetary scientist at Charles
University in Prague, told Space.com.

More data from the Cassini
spacecraft could help determine which of these models is
more likely, the researchers said. They detailed their findings
online in the July 6 edition of the journal Nature
Geoscience.

Understanding more about the geysers of Enceladus would shed
light on the alien moon's interior and on whether or not life
dwells there, said Cassini imaging team leader Carolyn Porco.

"All we have to do is fly through the plume with the proper
instrumentation and we could make great progress towards
answering the question, 'Is there biological
activity ?'" Porco told Space.com. "A group of us are in the
throes of designing a mission to do just that. Anything that
assists us in knowing how to plan such a mission is of vital
importance to our next exploratory steps at Enceladus.